Glyphosate is well known in the art as an effective post-emergent foliar-applied herbicide. In its acid form, glyphosate has a structure represented by formula (I): ##STR1##
and is relatively insoluble in water (1.16% by weight at 25.degree. C.). For this reason it is typically formulated as a water-soluble salt.
Monobasic, dibasic and tribasic salts of glyphosate can be made. However, it is generally preferred to formulate glyphosate and apply glyphosate to plants in the form of a monobasic salt. The most widely used salt of glyphosate is the mono(isopropylammonium), often abbreviated to IPA, salt. Commercial herbicides of Monsanto Company having the IPA salt of glyphosate as active ingredient include Roundup.RTM., Roundup.RTM. Ultra, Roundup.RTM. Xtra and Rodeo.RTM. herbicides. All of these are aqueous solution concentrate (SL) formulations and are generally diluted in water by the user prior to application to plant foliage. Other glyphosate salts which have been commercially formulated as SL formulations include the mono(trimethylsulfonium), often abbreviated to TMS, salt, used for example in Touchdown.RTM. herbicide of Zeneca.
Various salts of glyphosate, methods for preparing salts of glyphosate, formulations of glyphosate or its salts and methods of use of glyphosate or its salts for killing and controlling weeds and other plants are disclosed in U.S. Pat. No. 4,507,250 to Bakel, U.S. Pat. No. 4,481,026 to Prisbylla, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 3,977,860 to Franz, U.S. Pat. No. 3,853,530 to Franz, and U.S. Pat. No. 3,799,758 to Franz. The aforementioned patents are incorporated herein in their entirety by reference.
Among the water soluble salts of glyphosate known in the literature, but never used commercially before the priority filing date hereof, is the monoethanolammonium (MEA) salt, having a structure represented by formula (II): ##STR2##
in the ionic form predominantly present in aqueous solution at a pH of about 4. This salt is disclosed, for example, by Franz in U.S. Pat. No. 4,405,531 cited above, as one of a very long list of organic ammonium salts of glyphosate useful as herbicides, and, as an example of a monoalkanolammonium salt, is included among the "particularly preferred" compounds disclosed therein. An aqueous solution of the MEA salt of glyphosate can be prepared by reacting approximately equimolar amounts of glyphosate acid and monoethanolamine in an aqueous medium. The reaction is exothermic.
Very few herbicides have been commercialized as their MEA salts. Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) is formulated as its MEA salt in certain herbicide products sold by DowElanco under the trademark Lontrel.RTM..
Glyphosate MEA salt has a molecular weight of 230, very similar to that of glyphosate IPA salt (228).
Solubility of glyphosate MEA salt in water is believed not to be recorded in prior art but is readily determined by procedures familiar to those skilled in the art. Similarly, aqueous solutions of glyphosate MEA salt at concentrations greater than about 40% by weight are believed not to have been specifically disclosed, thus any unusual or unpredicted properties of such solutions have not been publicly known. Concentrations expressed as percent by weight herein relate to parts by weight of salt or acid equivalent per 100 parts by weight of solution.
It can now be disclosed that glyphosate MEA salt has been determined to have a solubility in pure water at 20.degree. C. of about 64% by weight, that is, about 47% glyphosate acid equivalent (a.e.) by weight. This is very similar to the solubility of the IPA salt. Thus a simple aqueous solution concentrate of glyphosate MEA salt can readily be provided at a concentration of, for example, 46% a.e. by weight, comparable to that commercially obtainable with glyphosate IPA salt, as in the aqueous solution concentrate available from Monsanto Company under the name MON 0139.
A major advantage of the IPA salt over many other salts of glyphosate has been the good compatibility in aqueous solution concentrate formulations of that salt with a wide range of surfactants. Glyphosate salts generally require the presence of a suitable surfactant for best herbicidal performance. The surfactant can be provided in the concentrate formulation, or it can be added by the end user to the diluted spray composition. The choice of surfactant has a major bearing on herbicidal performance. For example, in an extensive study reported in Weed Science, 1977, volume 25, pages 275-287, Wyrill and Burnside found wide variation among surfactants in their ability to enhance the herbicidal efficacy of glyphosate, applied as the IPA salt.
Beyond some broad generalizations, the relative ability of different surfactants to enhance the herbicidal effectiveness of glyphosate is highly unpredictable.
Surfactants tending to give the most useful enhancement of glyphosate herbicidal effectiveness are generally but not exclusively cationic surfactants, including surfactants which form cations in aqueous solution or dispersion at pH levels of around 4-5 characteristic of SL formulations of monobasic salts of glyphosate. Examples are long-chain (typically C.sub.12 to C.sub.18) tertiary alkylamine surfactants and quaternary alkylammonium surfactants. The term "alkyl" as conventionally used herein in describing surfactant structures includes unsaturated as well as saturated hydrocarbyl chains. An especially common tertiary alkylamine surfactant used in aqueous solution concentrate formulations of glyphosate IPA salt has been the very hydrophilic surfactant polyoxyethylene (15) tallowamine, i.e., tallowamine having in total about 15 moles of ethylene oxide in two polymerized ethylene oxide chains attached to the amine group as shown in formula (III): ##STR3##
where R is a mixture of predominantly C.sub.16 and C.sub.18 alkyl and alkenyl chains derived from tallow and the total of m+n is an average number of about 15.
For certain applications, it has been found desirable to use a somewhat less hydrophilic alkylamine surfactant, such as one having less than about 10 moles of ethylene oxide, as suggested in U.S. Pat. No. 5,668,085 to Forbes et al., for example polyoxyethylene (2) cocoamine. That patent discloses illustrative aqueous compositions comprising such a surfactant together with the IPA, ammonium or potassium salts of glyphosate.
A wide variety of quaternary ammonium surfactants have been disclosed as components of aqueous solution concentrate formulations of glyphosate IPA salt. Illustrative examples are polyoxyethylene (2) cocoammonium chloride, disclosed in European Patent No. 0 274 369, polyoxyethylene (15) cocoammonium chloride, disclosed in U.S. Pat. No. 5,317,003, and various quaternary ammonium compounds having formula (IV): EQU (R.sup.1)(R.sup.2)(R.sup.3)N.sup.+ --CH.sub.2 CH.sub.2 O--(CH.sub.2 CH(CH.sub.3)O).sub.n H Cl.sup.- (IV)
where R.sup.1, R.sup.2 and R.sup.3 are each C.sub.1-3 alkyl groups and n is an average number from 2 to 20, disclosed in U.S. Pat. No. 5,464,807.
PCT Publication No. WO 97/16969 discloses aqueous solution concentrate compositions of glyphosate, in the form of the IPA, methylammonium and diammonium salts, comprising a quaternary ammonium surfactant and an acid salt of a primary, secondary or tertiary alkylamine compound.
Other cationic surfactants which have been indicated as useful in aqueous solution concentrate compositions of glyphosate salts include those disclosed in PCT Publication No. WO 95/33379. It is further disclosed in PCT Publication No. WO 97/32476 that highly concentrated aqueous compositions of glyphosate salts can be made with certain of these same cationic surfactants, with the further addition of a defined component that enhances stability of the compositions. Glyphosate salts exemplified therein are the IPA salt and the mono- and diammonium salts.
Among amphoteric or zwitterionic surfactants reported to be useful components of aqueous solution concentrate formulations of glyphosate IPA salt are alkylamine oxides such as polyoxyethylene (10-20) tallowamine oxide, disclosed in U.S. Pat. No. 5,118,444.
Nonionic surfactants are generally reported to be less effective in enhancing herbicidal activity than cationic or amphoteric surfactants when used as the sole surfactant component of SL formulations of glyphosate IPA salt; exceptions appear to include certain alkyl polyglucosides, as disclosed for example in Australian Patent No. 627503, and polyoxyethylene (10-100) C.sub.16-22 alkylethers, as disclosed in PCT Publication No. WO 98/17109. Anionic surfactants, except in combination with cationic surfactants as disclosed in U.S. Pat. No. 5,389,598 and U.S. Pat. No. 5,703,015, are generally of little interest in SL formulations of glyphosate IPA salt.
Although some of the surfactant types mentioned above have been disclosed as being useful in compositions of glyphosate salts in general, none of them has been specifically disclosed in association with glyphosate MEA salt. Recently, a class of alkyletheramine, alkyletherammonium salt and alkyletheramine oxide surfactants has been disclosed in U.S. Pat. No. 5,750,468 to be suitable for preparation of aqueous solution concentrate formulations of various glyphosate salts, the MEA salt being included in the list of salts mentioned. It is disclosed therein, without reference to any particular glyphosate salt, that an advantage of the subject surfactants when used in an aqueous composition with glyphosate salts is that these surfactants permit the glyphosate concentration of the composition to be increased to very high levels. There is no suggestion, however, of any particular advantage in this regard when the glyphosate salt used is the MEA salt.
It is likely that serious consideration of glyphosate MEA salt as a herbicidal active ingredient has been inhibited hitherto by relative difficulty in formulating this salt as a highly concentrated SL product together with preferred surfactant types. As illustration of the problems involved in formulating glyphosate MEA salt with surfactants, the hitherto most widely used surfactant in glyphosate IPA salt compositions, namely polyoxyethylene (15) tallowamine of formula (III) above, is relatively incompatible in aqueous solution with glyphosate MEA salt.
For example, one convenient and practically useful indicator of surfactant/salt compatibility in concentrated aqueous solutions is "cloud point". This is a measure of the maximum temperature at which a given aqueous composition containing a surfactant and a salt at defined concentrations forms a single-phase solution. Above the cloud point, the surfactant separates from the solution, initially as a hazy or cloudy dispersion, and, upon standing, as a distinct phase generally rising to the surface of the solution. Cloud point of a composition is normally determined by heating the composition until the solution becomes cloudy, and then allowing the composition to cool, with agitation, while its temperature is continuously monitored. A temperature reading taken when the solution clears is a measure of cloud point.
A cloud point of 50.degree. C. or higher is normally considered acceptable for most commercial purposes for a glyphosate SL formulation. As shown in Table 1 below, an aqueous solution of glyphosate IPA salt at a concentration of 31% a.e. by weight can be loaded with up to 15% by weight of polyoxyethylene (15) tallowamine while maintaining a cloud point of 50.degree. C. or higher, whereas an aqueous solution of glyphosate MEA salt at the same a.e. concentration can tolerate only up to 4% by weight of the same surfactant without the cloud point falling significantly below 50.degree. C. At 8% or more by weight of polyoxyethylene (15) tallowamine, the surfactant is insoluble in a 31% a.e. by weight aqueous solution of glyphosate MEA salt, even at room temperature (20-25.degree. C.). A surfactant loading of 4% or lower in a concentrate composition having a glyphosate concentration of 31% a.e. by weight or higher is unlikely to provide the herbicidal effectiveness sought by the glyphosate herbicide user, except in situations where the herbicide is to be applied in an extremely low water volume such that a surfactant concentration of about 0.1% or higher is maintained in the diluted application composition.
TABLE 1 Cloud points of glyphosate salt solutions containing polyoxyethylene (15) tallowamine surfactant.sup.1 at various concentrations Surfactant Cloud point (.degree. C.) concentration (wt %) IPA salt, 31 wt % a.e. MEA salt, 31 wt % a.e. 2 88 67 4 84 49 6 79 45 8 75 insoluble at 20-25.degree. C. 10 70 insoluble at 20-25.degree. C. 15 50 insoluble at 20-25.degree. C. 20 40 insoluble at 20-25.degree. C. .sup.1 Ethomeen .TM. T/25 of Akzo
At glyphosate a.e. concentrations higher than 31% a.e. by weight, the amount of polyoxyethylene (15) tallowamine that can be included in a composition while maintaining an acceptably high cloud point is even lower.
It would be desirable to provide a storage-stable aqueous solution concentrate composition of glyphosate having an agronomically useful surfactant content, or that is "fully loaded" with surfactant, yet delivers to the weed control practitioner more glyphosate active ingredient per unit volume of concentrate than a comparably surfactant-containing or "fully loaded" composition based on glyphosate IPA salt.
An "agronomically useful surfactant content" means containing one or more surfactants of such a type or types and in such an amount that a benefit is realized by the user of the composition in terms of herbicidal effectiveness by comparison with an otherwise similar composition containing no surfactant. By "fully loaded" is meant having a sufficient concentration of a suitable surfactant to provide, upon conventional dilution in water and application to foliage, herbicidal effectiveness on one or more important weed species at least equal to that of current commercial glyphosate IPA salt products such as Roundup.RTM. herbicide, without the need for further surfactant to be added to the diluted composition.
By "storage-stable", in the context of an aqueous solution concentrate composition of glyphosate salt further containing a surfactant, is meant not exhibiting phase separation on exposure to temperatures up to about 50.degree. C. (i.e., the composition must have a cloud point of about 50.degree. C. or higher), and preferably not forming crystals of glyphosate or salt thereof on exposure to a temperature not lower than about 0.degree. C. for a period of up to about 7 days. Ideally the cloud point should be 60.degree. C. or higher, and the composition should withstand temperatures not lower than about -10.degree. C. for up to about 7 days without crystal formation, even in the presence of seed crystals of the glyphosate salt.
A surfactant that is described herein as "compatible" with a glyphosate salt at specified surfactant and glyphosate a.e. concentrations is one that provides a storage-stable aqueous solution concentrate as defined immediately above containing that surfactant and salt at the specified concentrations.
Users of liquid herbicidal products typically meter the dosage by volume rather than by weight, and such products are usually labeled with directions for suitable use rates expressed in volume per unit area, e.g., liters per hectare (l/ha) or fluid ounces per acre (oz/acre). Thus the concentration of herbicidal active ingredient that matters to the user is not percent by weight, but weight per unit volume, e.g., grams per liter (g/l) or pounds per gallon (lb/gal). In the case of glyphosate salts, concentration is often expressed as grams of acid equivalent per liter (g a.e./l).
Historically, surfactant-containing glyphosate IPA salt products such as Roundup.RTM. and Roundup.RTM. Ultra herbicides of Monsanto Company have most commonly been formulated at a glyphosate concentration of about 360 g a.e./l. The surfactant-containing glyphosate TMS salt product Touchdown.RTM. of Zeneca has been formulated at a glyphosate concentration of about 330 g a.e./l. Products at lower a.e. concentration, i.e., more dilute, are also sold in some markets, but carry a cost penalty per unit of glyphosate they contain, primarily reflecting packaging, shipping and warehousing costs.
Further benefits in cost saving and in convenience to the user are possible if a "fully loaded" aqueous solution concentrate composition, or at least one having an agronomically useful surfactant content, can be provided at a glyphosate concentration significantly higher than 360 g a.e./l, for example about 420 g a.e./l or higher, or even about 480 g a.e./l or higher.
At very high glyphosate a.e. concentrations such as these, a significant problem normally occurs. This is the difficulty in pouring and/or pumping of the aqueous solution concentrate arising from the high viscosity of the concentrate, especially as manifested at low temperatures. It would therefore be highly desirable to have a highly concentrated aqueous solution of glyphosate salt that is less viscous than an aqueous solution of the IPA salt at the same glyphosate a.e. weight/volume concentration.
As will be clear from the disclosure that follows, these and other benefits are provided by the present invention.